Modern power supplies use the basic technology of switching power converters to improve efficiency and compactness. The advent of the silicon controlled rectifier has allowed these switching power converters to be solid state devices. Variable AC or DC output voltages or currents in these switching power supplies are generated by many well known methods, most commonly by the method known as pulse width modulation ("PWM"). Typical applications include AC power supplies for AC motor drives or DC power supplies for a myriad of electronic circuits or industrial processes.
Using the pulse width modulation method, solid state switches within converters are switched at relatively high frequencies. The variation of the switching intervals (pulse width) is controlled to produce the desired slow variations of the average DC or AC output waveforms. However, a non-negligible amount of energy loss is associated with each switching action of the solid state switches. Because switching at high frequencies is required to achieve the desirable output waveform fidelities, the overall switching losses of conventional converters are rather high, since energy is lost during each switching action. Consequently, conventional converters are limited to lower power applications, where the resulting low efficiencies and heat dissipation problems caused by the losses in the switches are tolerable.
An example of a conventional power converter, known as the Cuk converter, has been disclosed by S. Cuk and R. D. Middlebrook, Coupled Inductor and Other Extensions of a New Optimum Topology Switching DC-to-DC Converter, IEEE Industry Application Society Annual Meeting, pp. 1110-1126, 1977. Significant limitations exist within the Cuk converter and other conventional converters because the switching elements are "hard switched". Hard switching occurs when the switches within the converter are switched while there is a voltage across the switches and a current flowing through the switches. Because of this hard switching, significant losses, as described above, result within the switches. Consequently, the useful power of the Cuk and other conventional converters is limited to less than 10 kilowatts, and their frequency range is also limited because of switching losses to ranges on the order of 10 kilohertz. Furthermore, because of the design and method of operation of the Cuk converter, only gate turn off devices, and not devices such as silicon controlled rectifiers, can be used as the switching elements.
Recent attention has focused on converters that use zero-current or zero-voltage switching for increasing power converter efficiency. Resonant and quasi-resonant converters have been developed to make use of these zero-current and zero-voltage switching techniques. However, difficulties involved in matching the operating frequency with the resonance components, magnetic saturation, and increased component stress present significant limitations for such converters.
Zero voltage "soft-switching" converters have also been proposed to overcome some of the limitations of resonant and quasi-resonant converters. See, S. Hamada et al., A New Conceptional PWM DC-DC Converter with Zero-Voltage Switching Incorporating Non-Controlled Saturable Reactors, IEEE-PESC Conf. Rec. 1989; J. A. Ferreira, et al., A General Soft Switching Converter Topology with a Parallel Nonlinear Network for High Power Applications, IEEE-PESC Conf. Rec. 1990; R. W. DeDoncker, et al., A Three-Phase Soft-Switched High-Power Density DC--DC Converter for High Power Applications, IEEE Trans. on IAS, Volume 27, No. 1, January-February 1991. However, these converters present significant limitations. In particular, they require increased current ratings, as high as 200 percent, especially in high power applications. The increased current results in significant increases in conduction losses, as well as higher power device costs.
Therefore, a need has arisen for a zero-loss switching converter that produces less device and component stresses than existing converter topologies. Furthermore, a need has arisen for a non-resonant converter that uses zero-current soft switching. A need has also arisen for a converter that can use both gate turn-off switches for efficient high frequency switching and SCRs for high power applications.